Bonded fiber optic gyro sensor coil including voids

ABSTRACT

A sensor coil for a fiber optic gyroscope. A continuous optical fiber is coated with bonding material of predetermined composition such as a thermoplastic material that exhibits a bond strength of at least 100 p.s.i. in tension, equivalent to at least 40 p.s.i. in shear. The coated fiber is arranged into a predetermined winding pattern whereby contacting portions of adjacent coil turns are bonded to one another. Void spaces between contacting portions of bonded turns reduce the volume of bonding material employed (as opposed to a filled potted sensor coil). The coil does not experience the differential in expansion along the axial and radial directions to the degree that has, for example, required the use of single flanged spools in the prior art. Its use with dual-flanged spools permits mounting arrangements that promote coil stiffness, thereby enhancing gyro performance in the presence of vibration.

BACKGROUND

1. Field of the Invention

The present invention relates to fiber optic gyroscopes. Moreparticularly, this invention pertains to a sensor coil that is lesssensitive to the stiffness and thermal properties of bonding agents thanpotted designs.

2. Description of the Prior Art

An interferometric fiber optic gyroscope comprises the following maincomponents: (1) a light source, (2) two beamsplitters (fiber opticdirectional coupler and/or integrated-optics Y-junctions) to satisfy therequirement of a "minimum reciprocal configuration" (S. Ezekiel and M.J. Arditty, Fiber Optic Rotation Sensors New York, Springer-Verlag p.2-26 1982), (3) a fiber sensing optic coil made of either polarizationmaintaining (PM) fiber or made of low-birefringence (standardtelecommunications) fiber, (4) a polarizer (and sometimes one or moredepolarizers), and (5) a detector. Light from the light source is splitby the loop beamsplitter into copropagating and counterpropagating wavestravelling in the sensing coil. The associated electronics measures thephase relationship between the two interfering, counterpropagating beamsof light that emerge from opposite ends of the coil. The differencebetween the phase shifts experienced by the two beams is proportional tothe rate of rotation of the platform to which the instrument is fixed,due to the well-known Sagnac effect.

Environmental factors can affect the measured phase shift differencebetween the counterpropagating beams, thereby introducing a bias error.Such environmental factors include variables such as temperature,vibration (acoustical and mechanical) and magnetic fields. In general,such factors are both time-varying and unevenly distributed throughoutthe coil. These environmental factors induce variations in the opticallight path that each counterpropagating wave encounters as it travelsthrough the coil. The phase shifts induced upon the two waves areunequal, producing a net undesirable phase shift which isindistinguishable from the rotation-induced signal.

One approach to attain a reduction of sensitivities arising fromenvironmental factors has involved the use of various symmetric coilwinding configurations. In such coils, the windings are arranged so thatthe geometrical center of the coil is located at the innermost layerwhile the two ends of the coil are located at the outermost layers.

N. Frigo has proposed the use of particular winding patterns tocompensate for non-reciprocities in "Compensation of Linear Sources ofNon-Reciprocity in Sagnac Interferometers". Fiber Optics and LaserSensors I, Proc. SPIE Vol. 412 p. 268 (1983). Furthermore, U.S. Pat. No.4,793,708 of Bednarz entitled "Fiber Optic Sensing Coil" teaches asymmetric fiber optic sensing coil formed by dualpole or quadrupolewinding. The coils described in that patent exhibit enhanced performanceover the conventional helix-type winding.

U.S. Pat. No. 4,856,900 of Ivancevic entitled "Quadrupole-Wound FiberOptic Sensing Coil and Method of Manufacture Thereof" teaches animproved quadrupole-wound coil in which fiber pinching and microbendsdue to the presence of pop-up fiber segments adjacent the end flangesare overcome by replacing such pop-up segments with concentrically-woundwalls of turns for climbing between connecting layers. Both of theaforementioned United States patents are the property of the assigneeherein.

While appropriate coil winding techniques minimize some of the biaserrors found in the output of a fiber optic gyro, they are not capableof eliminating all of such biases. In particular, the design of the gyrosensor coil can impact the gyro's random walk, bias stability, biastemperature sensitivity, bias temperature-ramp sensitivity, biasvibration sensitivity, bias magnetic sensitivity, scale factortemperature sensitivity and input axis temperature sensitivity.

The relatively-high proportion of sensor coil volume consumed by theadhesive material of a potted sensor coil (about twenty-five percent)means that the physical parameters of the potting material play a largerole in determining gyro performance. FIG. 1 is a cross sectional viewof a portion of such a potted sensor coil. As can be seen, the turns ofa continuous optical fiber 10 are encapsulated within a surroundingmatrix of potting material 12, forming an integral structure therewith.It has been well-recognized that potting is advantageous forfacilitating the precision of coil winding. Furthermore, it is disclosedin U.S. patent Ser. No. 5,371,593 of co-inventors Amado Cordova, DonaldJ. Bilinski, Samuel N. Fersht, Glenn M. Surabian, John D. Wilde and PaulA. Hinman entitled "Sensor Coil For Low Bias Fiber Optic Gyroscope",that the composition of the potting material can have a significantimpact upon the vibration bias sensitivity of the fiber optic gyro as aresult of changes in fiber length and refractive index brought about byvibration dynamic strains.

The referenced United States patent discloses a sensor coil whose designincorporates a number of features for minimizing the aforesaidenvironmental factors. Among the issues identified and addressed in thatpatent is the existence of a relationship between the modulus ofelasticity of the potting material and vibration-induced bias.Generally, gyro performance (in terms of vibration) is significantlyimproved by potting material possessing a high modulus of elasticity(Young's modulus). The modulus should not, however, be so high as toproduce other problems related to gyro operation at temperaturessignificantly removed from the curing temperature of the pottingmaterial. Such problems include temperature related coil cracking,h-parameter (polarization cross-coupling) degradation of coilsfabricated of PM-fiber, and large bias temperature-sensitivity. PendingU.S. patent application Ser. No. 08/266,993 of Amado Cordova and GlennM. Surabian, property of the assignee herein, teaches the selection ofpolymer potting materials based, in part, upon the relationship betweenthe glass transition temperature of a candidate material and theoperational temperature range of the gyro.

Pending U.S. patent application Ser. Nos. 08/526,725 of Ralph A.Patterson and 08/299,585 of Donald J. Bilinski, Gene H. Chin, AmadoCordova and Samuel N. Fersht, each property of the assignee herein,disclose spools for gyro sensor coils designed to overcome otherproblems associated with the properties of adhesive materials when usedto encapsulate coils. A substantial differential is generally observedto exist between the radial and axial thermal expansion characteristicsof an encapsulated potted sensor coil. Typically, the radial thermalexpansion coefficient of a potted sensor coil is less than 10 parts permillion (ppm) per degree Centigrade (deg C.), whereas the axial thermalexpansion coefficient of a potted coil is typically larger than 200 ppmper deg C. This large anisotropy of the thermomechanical properties of apotted coil results from the large differences that exist between thethermal expansion properties of the glass fiber (cladding and core) andthe surrounding polymers (in particular, the potting material) and thefact that the fiber turns are fully encapsulated by the pottingadhesive, leaving no void spaces therebetween. The net result is thatradial expansion of the potted coil is fully determined by thelow-expansion, stiff glass fiber turns whereas the axial expansion isdetermined by the high-expansion, softer potting material.

Each of the above-mentioned pending applications discloses a spool orcoil mount that includes a single support flange. This is in contrast tospools employing paired end flanges. The single flange arrangementspermit the potted coil to be axially unconstrained, thereby preventingexcessive stress on the optical fiber, and consequent detrimentaleffects on gyro bias performance in response to temperature changes. Thepotted coil would otherwise be prone to such excessive stress due to thegreater degree of expansion of the filled coil than of the spool in theaxial direction. On the other hand, coil mounts or spools of the typethat employ a pair of end flanges (necessarily constraining the axialdimension of the coil) add highly desirable coil stiffness that cansignificantly reduce the gyro vibration sensitivity. Therefore, areduction in the coil's thermally-induced axial expansion will increasethe feasibility of spool mounts that include paired end flanges, leadingto improved gyro performance, particularly with respect to vibrationsensitivity.

SUMMARY OF THE INVENTION

The foregoing and additional shortcomings and disadvantages of the priorart are addressed by the present invention that provides, in a firstaspect, a sensor coil for a fiber optic gyroscope. Such a coil includesa continuous optical fiber. The fiber is arranged into a plurality ofconcentric cylindrical layers with adjacent layers contacting oneanother. Each of the layers comprises a plurality of turns arranged in apredetermined winding pattern with adjacent turns contacting oneanother. The optical fiber is coated with material of predeterminedcomposition for bonding adjacent turns and layers to one another atpoints of contact.

In a second aspect, the invention presents a sensor coil for a fiberoptic gyroscope that includes a continuous optical fiber coated with alayer of bonding material and arranged onto a support spool in aplurality of contacting, concentric cylindrical layers of turns. Eachturn is arranged into a predetermined winding pattern with adjacentturns contacting one another so that turns are bonded to one another atthe points of contact and void spaces remain therebetween.

In a third aspect, the invention provides a method for forming a sensorcoil for a fiber optic gyroscope. Such method is begun by coating anoptical fiber with a layer of predetermined bonding material. The fiberis then wound into a predetermined pattern upon a spool and curedwhereby the contacting portions of turns of the wound fiber are bondedto one another and void spaces remain therebetween.

The preceding and other features and advantages of the invention willbecome further apparent from the detailed description that follows. Suchdescription is accompanied by a set of drawing figures. Numerals of thedrawing figures, corresponding to those of the written text, point tothe various features of this invention with like numerals referring tolike features throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a representative portion of thelayered windings of a potted sensor coil in accordance with the priorart wherein the windings thereof are encapsulated within an adhesivepotting material to form a filled structure;

FIG. 2 is a perspective view of a sensor coil for a fiber opticgyroscope in accordance with the invention mounted to a spool of thetype that includes paired end flanges; and

FIG. 3 is a cross sectional view of a portion of a fiber optic sensorcoil in accordance with the invention in which adjacent turns are coatedwith bonding material and fused to one another at points of mutualcontact.

DETAILED DESCRIPTION

Turning to the drawings, FIG. 2 is a perspective view of a sensor coil20 in accordance with the present invention. As mentioned earlier, thesensor coil 20 provides a critical element of a fiber optic gyro. Inuse, it is rigidly fixed to a navigation system platform, serving tosense platform rotation rate. The rotation rate information is employedby an aircraft flight computer to perform an analytical "leveling" ofthe navigation platform. In this way, the outputs of the I.N.S.accelerometers are converted to true measures of velocity and distancealong the inertial axes.

The sensor coil 20 comprises a continuous optical fiber 22 that is woundupon a supportive spool 24 and serves as an optical guide for receivinga counterpropagating beam pair emitted from a common light source (notshown). The supportive spool 24 of FIG. 2 preferably includes a pair ofend flanges 26 and 28 as shown. Unlike potted sensor coil structures inwhich an adhesive material encapsulates and fills the voids around thewound fiber, the use of bonding material is sufficiently restricted tolessen the impact of the physical properties of the material upon theperformance of the wound coil. As one consequence, the coil 20 will notexperience differential expansions between axial and radial directionsof such a magnitude as to require the use of single flange spooldesigns. As will be seen below, the coil of the present invention issuitable for use with dual-flanged mounting spools that add desirablestiffness under vibration.

FIG. 3 is a cross-sectional view of a portion (actually a limited numberof adjacent turns of the optical fiber 22) of the fiber optic sensorcoil 20 of the invention taken at the section 3 of FIG. 2. As shown, thecontinuous fiber 22 includes a central cladding 32 of glass. Typicallythe cladding 32 is between 80 and 125 microns in diameter with a centralcore 34 of higher index of refraction glass. Representative corediameters range from 5 to 8 microns. An outer jacket 36 of polymercomposition protects the glass cladding 34. The jacket 36 may consist ofoverlying resin coatings, for example, an inner coating of a resin ofrelatively low-Young's modulus and an outer coating of a resin ofrelatively high Young's modulus. Typically, each of the inner and outerresin coatings is about 20 microns thick. The jacket 36 may also consistof a single resin coating of an intermediate value of Young's modulus.

Finite element modelling (FEM) of the fiber gyro sensor coil has beenemployed to calculate the maximum environmental stresses experienced bythe different coil constituents. In particular, FEM has determined themaximum radial stress (perpendicular to the fiber axis) that tends toseparate the potting or bonding material from the fiber jacket. Suchmaximum de-bonding stress has been found to be about 100 pounds persquare inch (p.s.i) for the high-accuracy coils of the aforementionedpatent applications of Patterson and of Bilinski et al. Consequently,the minimum bond strength in tension of the bonding material of thepresent invention to itself and to the fiber jacket must be at least 100p.s.i. Measurements of bond strength in tension have been performedemploying the American Standard Test Method (ASTM) number D897 "TensileProperties of Adhesive Bonds." In such method, thin films (about 100 to200 microns thick) of the fiber jacket material are sandwiched betweenthin films of the potting or bonding material (in liquid or uncuredform) and metallic samples fabricated according to the ASTM. The pottingor bonding material is then cured. "Sandwich" samples are then pulled intension. The force required to break the bond is recorded and the bondstrength in tension is given as the ratio of such force to contact area.It is important that failure (or breakage of the bond) occurs at thepotting or bonding material-to-fiber jacket film interface. It was foundexperimentally that a bond strength, in tension, of the potting orbonding material to itself and to the fiber jacket film of 100 p.s.i ormore is adequate for fiber gyro coils. An alternate method to measurebond strength is in shear rather than tension, utilizing a short segmentof optical fiber rather than a thin film of the fiber jacket material.In the alternate method, the fiber is embedded in the potting or bondingmaterial and sandwiched between pieces of quartz. The preferred fiberlength is 1/2 inch. After curing, the fiber is pulled in the directionof its axis while the pieces of quartz are held until the bond at thepotting or bonding material-to-fiber jacket interface breaks. The bondstrength in shear is given by the breaking force divided by the lateralcontact area of the fiber (i.e., fiber length times fiber diameter timesπ). It was found experimentally that a bond strength in shear of thepotting or bonding material to itself and to the fiber jacket of 40p.s.i or more is adequate for fiber gyro coils. Thus 100 p.s.i. or morein tension is equivalent to 40 p.s.i. or more in shear for the purposesof this invention.

The exterior of the optical fiber 22 is coated with a layer 38 ofmaterial for bonding the turns of the coil 20 in accordance with thepresent invention. The layer 38, preferably less than 10 microns inthickness, is fabricated of material selected so that, when cured, abond strength, in tension, of at least 100 p.s.i. (alternatively atleast 40 p.s.i. in shear) is provided between contacting turns. Anexample of an appropriate bonding material for forming the coating layer38 is the thermoplastic polymer coating commercially available under thetrademark KYNAR or PVDF (polyvinylidene fluoride) from PolymicroTechnologies Inc. of Phoenix, Ariz. The optical fiber may be coated by adip-coating process (solution coating) or an extrusion process (hot meltcoating). Dip-coating permits excellent dimensional control and goodbonding to the fiber acrylate material. Once the fiber gyro coil iswound, the bonding material is activated to bond adjacent fiber turns.KYNAR or PVDF can be activated by heating at about 160 deg C. for alimited period of time to avoid compromising optical fiber strength.Another appropriate bonding material is the thermoplastic materialcommercially available under the trademark BUTVAR (polyvinyl butyral)from Shawinigan Ltd. of London, England. This resin is widely used as awire enamel to bond electronic coils and bobbins and is produced by thereaction between polyvinyl alcohol and aldehydes. A third example of anappropriate bonding material for forming the layer 38 is thethermoplastic wire enamel commercially available under protectdesignation VG-8637 from Supreme Resources, Inc. of New York, N.Y. Thismaterial, based on an epoxy resin solution in a conventional solventsystem (cresylic acid and aromatic hydrocarbon), can withstand highbaking temperatures without loss of bonding properties while maintainingexcellent coatability over a wide bake range. It can be activated(cured) by either heat or solvent. Temperatures of 150 degreesCentigrade or more are suggested for thermal bonding while therecommended solvents include acetone and methyl ethyl ketone.

The concept of a bonded, as opposed to an encapsulated potted sensorcoil, frees the designer to a large extent from design considerationsbased upon and substantially limited by the thermal properties of theadhesive potting material. As mentioned above, such material parametersas Young's modulus, glass transition temperature and coefficient ofthermal expansion of the potting material have so significantly affectedgyro performance in encapsulated coil gyros as to virtually dictate theuse of a particular potting material. Whereas between 25 and 30 percentof the volume of an encapsulated (i.e. filled) potted coil comprises theadhesive material, a coil in accordance with the invention formed of awound continuous optical fiber with a coating thickness of 10 microns orless of appropriate bonding material comprises approximately 5 to 10percent bonding material with the remaining 15 to 25 percent of coilvolume consisting of inter-turn void spaces 40.

The reduction in the amount of bonding material employed necessarilyreduces the impact of the thermal and mechanical properties of thebonding material upon gyro performance and consequently reduces theeffect of the bonding material upon coil, spool and mount designs. Thevoid spaces 40 of the "honeycombed" cross section of the sensor coilfurther reduce the effect of the thermal characteristics of the bondingmaterial upon gyro performance. Such void spaces 40 provide a built-in"margin of error" by creating areas into which the bonding material 38as well as the jacket 36 may expand in the presence of coil heatingwithout excessively increasing the size (principally, the axialdimension) of the sensor coil. Thus, the sensor coil 20 of the inventionis physically quite stable, as is the gyro performance, in the presenceof thermal cycling.

The sensor coil of the invention may be formed by first drawing theoptical fiber through a bath of appropriate bonding material such as theabove-described thermoplastics (KYNAR, BUTVAR or VG-8637). After removalfrom the bath, the coated fiber is then preferably advanced onto a pairof spools in equal lengths that act as supply spools for a quadrupolewinding process. As mentioned earlier, such a winding pattern minimizesthe amount of Shupe effect-induced bias. Heat may be applied to the coilwindings for activating the bonding material either as the coatedoptical fiber is being wound upon a take-up spool or after thecompletion of winding. Upon activation of the bonding material, turns ofthe optical fiber are fused to one another at points of mutual contact.As noted earlier, the particular bonding materials referenced above areheat-curable at 150 to 160 degrees Centigrade. However, by extending theduration of the heating process, the requisite bonding strength may beobtained at 120 degrees Centigrade. Such lower curing temperature ispreferred in view of the thermal characteristics of the polymer materialthat serves as an outer protective jacket of the optical fiber.

By applying the activating heat as the coil is wound, one may fix thelocations of turns with great precision. This avoids the problem of"slumping" that is commonly observed in wound coils. Slumping refers tothe tendency of some turns to migrate from a particular layer towardunderlying layers. By fixing (i.e. bonding) the position of a turn as itis wound, post-winding slumping (which degrades Shupe bias performance)is avoided.

Thus it is seen that the present invention provides an improved bondedsensor coil for a fiber optic gyroscope. The coil is substantially lesssubject to the effects of temperature variations than is a conventionalpotted sensor coil of the filled type. By reducing the volume of bondingmaterial employed, inter-turn void spaces are created. Such void spacesare capable of absorbing thermally-induced expansion of the coil bondingmaterial and the polymer jackets. The combination of a reduction in thevolume of bonding material coupled with the creation ofexpansion-absorbing void spaces presents the gyro designer with numerousoptions, including the utilization of highly desirable support spools ofthe paired-end flange type, not available for high accuracy performancegyros that incorporate potted coils of the filled type.

While this invention has been described with reference to its presentlypreferred embodiment, it is not limited thereto. Rather, this inventionis limited only insofar as defined by the following set of patent claimsand includes within its scope all equivalents thereof.

What is claimed is:
 1. A sensor coil for a fiber optic gyroscopecomprising, in combination:a) an optical fiber; b) said fiber beingarranged into a plurality of concentric cylindrical layers with adjacentlayers contacting one another; c) each of said layers comprising aplurality of turns of said fiber; d) said turns being arranged into apredetermined winding pattern with adjacent turns contacting oneanother; e) terminal ends of said wound fiber being remote from theinner layer of said coil; f) each of said turns being bonded to oneanother by a predetermined bonding material at said points of contact;and g) said wound coil being mounted between opposed flanges of a spool.2. A sensor coil as defined in claim 1 wherein said bonding material isof thermoplastic composition characterized by a bond strength to itselfand to the jacket of said fiber equivalent to at least 100 p.s.i. intension.
 3. A sensor coil as defined in claim 2 wherein said turns arearranged into a quadrupole winding pattern.
 4. A sensor coil as definedin claim 2 wherein said bonding material is selected from the groupconsisting of polyvinylidene fluoride, polyvinyl butyral and wireenamel.
 5. A sensor coil for a fiber optic gyroscope comprising, incombination:a) a continuous optical fiber coated with a layer of bondingmaterial and arranged onto a support spool including a pair of endflanges in a plurality of contacting concentric cylindrical layers ofturns; and b) each turn being arranged into a predetermined windingpattern with adjacent turns contacting one another so that said turnsare bonded to one another at said points of contact and void spacesremain between non-contacting portions of said coated fiber.
 6. A sensorcoil as defined in claim 5 wherein said turns are arranged into aquadrupole winding pattern.
 7. A sensor coil as defined in claim 5wherein said bonding material further comprises:a) said bonding materialis of thermoplastic composition; and b) said thermoplastic ischaracterized by a bond strength to itself and to the jacket of saidfiber equivalent to at least 100 p.s.i. in tension.
 8. A sensor coil asdefined in claim 7 wherein said bonding material is selected from thegroup consisting of polyvinylidene fluoride, polyvinyl butyral and wireenamel.
 9. A method for forming a sensor coil for a fiber opticgyroscope comprising the steps of:a) coating an optical fiber with alayer of predetermined bonding material of no greater than 10 micronsthickness selected from the group consisting of polyvinylidene fluoride,polyvinyl butyral and wire enamel; then b) winding said optical fiberinto a predetermined gyro coil pattern upon a spool including opposedend flanges; and c) curing said bonding material whereby contactingportions of turns of said wound fiber are bonded to one another and voidspaces remain between said bonded portions.